1. Field of the Invention
The present invention relates to an objective optical system.
2. Description of Related Art
Conventionally, microscopic observation using an objective optical system is widely used. For example, in order to observe the behavior or the like of biological molecules in cells and tissue of cultured organisms using images, a method in which specific molecules, tissue, cells, or the like tagged with a colorant or fluorescent marker are observed with a fluorescence microscope, a confocal laser scanning microscope, or the like is known. In recent years, because the behavior of molecules of living mammal organisms, such as mice, is sometimes different from that of cultured cells, various in vivo imaging techniques, which enable observation of biological molecules in the living tissue and cells while the living organism is alive (in vivo), have been proposed. Furthermore, in order to observe a living organism in a minimally invasive manner, there has been proposed a microscope that employs an objective lens consisting of small-diameter optical systems as the objective lens of the microscope so that the objective lens can be directly inserted into the living organism to observe the living organism in a minimally invasive manner (for example, see Japanese Unexamined Patent Application, Publication No. 2006-119300).
Conventional microscopes, such as laser-scanning confocal microscopes, are not intended for the observation of the organs of living (in vivo) small laboratory animals, such as rats and mice. However, when the inside of a living organism is to be observed, because the conventional objective lens of the microscope has a large outside diameter, a wide incision needs to be made in the living body for observation. However, because such a wide incision of the living body is highly invasive, long-term observation is impossible.
That is, in order to observe the organs of such small laboratory animals, the skin and muscle tissue need to be incised or the cranium needs to be drilled to expose the internal organs. However, because the size of the objective lens to be disposed close to the observation area is large relative to the small laboratory animal or the observed object, when the internal organs or the like are to be observed, the skin, the muscle tissue, or the like need to be widely incised or provided with a large hole.
On the other hand, Japanese Unexamined Patent Application, Publication No. 2006-119300 discloses an optical system having a small diameter at the tip. However, this is still highly invasive when a deep part of a relatively small organ, such as the brain, of a mouse is to be observed, and, considering the damage to the living body, there is an inconvenience in that observation under normal conditions is difficult. Furthermore, this optical system has problems not only in that it has a small numerical aperture for observation using multiphoton excitation, decreasing the resolution, but also in that it has a poor S/N due to weak detected light.
Moreover, when a deep part of a relatively small organ, such as the brain, of a mouse or a deep part of a living body portion susceptible to damage due to invasion is to be observed with a conventional objective optical system, such as one disclosed in Japanese Unexamined Patent Application, Publication No. 2006-119300, an invasive portion of the objective optical system needs to be designed as thin as possible. However, there is an inconvenience in that the more the diameter of the tip of the objective lens is reduced, the more the field of view (FOV) is narrowed.
In order to microscopically observe living tissue, such as cells and muscles, or various organs, such as the heart and the liver, in particular, the brain tissue, of living mammals, such as small laboratory animals, for a relatively long time in a minimally invasive manner, the diameter of the tip needs to be further reduced. However, further reduction in conventional small-diameter objective lens increases the occurrence of aberrations, such as spherical aberration and field curvature, causing an inconvenience when microscopic observation is performed. This causes a similar inconvenience when observation is to be performed with a wider field of view using the conventional small-diameter objective lens.